1. Field of the Invention
The present invention relates to an antenna duplexer. More specifically, the present invention relates to an antenna duplexer used for mobile communication equipment, such as automobile telephones and portable telephones.
2. Description of the Background Art
FIG. 1 is an equivalent circuit diagram of an antenna duplexer serving as an antenna both for a transmitter and for a receiver. Referring to FIG. 1, the antenna duplexer includes three terminals for input and output, which are designated as transmitting terminal TX, receiving terminal RX and antenna terminal ANT. Resonators R1 and R2 are for the transmitter, and resonators R3 and R4 are for the receiver. Each of these resonators is connected to ground. Also, resonator R1 is connected to transmitting terminal TX through an external coupling capacitance Ce1, resonator R2 is connected to antenna terminal ANT through external coupling capacitance Ce2, resonator R3 is connected to antenna terminal ANT through external coupling capacitance Ce3, and resonator R4 is connected to receiving terminal RX through an external coupling capacitance Ce4.
FIG. 2 is a perspective view showing a specific example of the antenna duplexer schematically shown in FIG. 1. Referring to FIG. 2, the antenna duplexer includes two dielectric filters 1a and 1b, and a coupling board 20. Each of the dielectric filters 1a and 1b consists of two stages of resonators. More specifically, dielectric filter 1a includes an approximately rectangular dielectric block 10a which includes two resonator holes 21a and 22a extending from an apertured surface 11a to an opposite surface 12a. Also, inner conductors 31a and 32a are formed on inner peripheral surfaces of resonator holes 21a and 22a, respectively.
A pair of input/output electrodes 51a and 52a are formed at opposite corner portions of dielectric block 1a, and extend from a side surface to the bottom surface of dielectric block 1a. On the outer peripheral surface except the regions on which input/output electrodes 51a and 52a are formed, an outer conductor 4a is provided. The inner conductors 31a and 32a are not provided at end portions of resonator holes 21a and 22a on the side of the apertured surface (hereinafter referred to as "open end surface") 11a. Therefore on open end surface 11a, inner conductors 31a and 32a are isolated from external conductor 4a, i.e., not electrically conducted. On the other surface (hereinafter referred to as "short-circuited surface") 12a opposite to the open end surface 11a, the resonator holes 21a and 22a are electrically conducted to with the external conductor 4a (short-circuited).
The other dielectric filter 1b is formed similarly as the above described dielectric filter 1a. Namely, it includes a dielectric block 10b, resonator holes 21b and 22b, inner conductors 31b and 32b, an outer conductor 4b, input and output electrodes 51b and 52b, an open end surface 11b and a short-circuited end surface 12b.
The coupling board 20 is for coupling two dielectric filters 1a and 1b placed parallel to each other, and it includes input/output electrodes 201 and 202 and an antenna electrode 203 formed on the surface thereof. Input/output electrode 201 corresponds to the input/output electrode 51a of dielectric filter 1a, input/output electrode 202 corresponds to the input/output electrode 52b of the dielectric filter 1b, and antenna electrode 203 corresponds to input/output electrodes 52a and 51b of the dielectric filters 1a and 1b, respectively. On the entire surface of coupling board 20, except the regions where input/output electrodes 201, 202 and antenna electrode 203 are formed, a ground conductor 204 is formed.
The dielectric filters 1a and 1b, as described above, constitute filters each having two stages of resonators, by the coupling of the resonators formed in resonator holes 21a and 22a, and 21b and 22b, respectively. Comparing FIG. 1 to FIG. 2, resonators R1 and R2 shown in FIG. 1 represent resonators formed by resonator holes 21a and 22a, respectively, of dielectric filter 1a, while resonators R3 and R4 represent resonators formed by resonator holes 21b and 22b, respectively, of dielectric filter 1b. The external coupling capacitance Ce1 between resonator R1 and transmitting terminal TX, the external coupling capacitance Ce4 between resonator R4 and receiving terminal RX, and external coupling capacitances Ce2 and Ce3 between resonators R2 and R3 and antenna terminal ANT respectively, are provided by interelectrode capacitances formed between input/output electrodes 51a, 52a, 51b, 52b and corresponding inner conductors 31a, 32a, 31b, and 32b of the dielectric filters 1a and 1b.
However, in the conventional antenna duplexer shown in FIG. 2, two dielectric filters 1a and 1b formed by two dielectric blocks 10a and 10b, as well as a coupling board 20 for connecting, fixing and mounting the filters, are necessary for forming the antenna duplexer. This conventional antenna duplexer therefore requires a large number of parts and numerous assembly steps including the soldering of these components. Thus, the conventional antenna duplexer of FIG. 2 impedes reduction in size, and increases the cost of components, the number of manufacturing steps and the cost of manufacturing.
Other known examples of a conventional antenna duplexer includes a number of dielectric resonators each having one resonator hole formed in one dielectric block, and arranged parallel to each other. In such an example, external components such as capacitor elements are necessary, in a addition to the coupling board, which results in larger number of parts.